In the conventional in-situ testing method, a small-diameter bore-hole was produced down to a certain depth from the ground surface, and a cylindrically shaped zonde (inflatable membrane) was lowered down into the bore-hole. The zonde was then inflated and applied the lateral pressure to the surrounding circular wall of the soil deposit, based on the measurements of lateral pressure and radial displacement, the static strength and deformation properties of the soil deposit were inferred.
However, the conventional testing method was intended only to infer the static properties of the soil deposit, and the evaluation of the dynamic properties of the soil deposit, such as those under earthquake loading, traffic loading and machinery vibrations, was out of scope for its use. In the event of earthquakes, however, the strains gradually accumulate locally within the soil deposit leading in some cases to failures, due to the external forces associated with earthquakes, which might be even lower than the collapse loads. It is extremely important therefore to explore the in-situ dynamic properties of soils. The seismic excitations involve complex characteristics and it has not reached a point where the evaluation of the dynamic properties of soils under such complex seismic excitations is properly established.
Among the conventional assessments of liquefaction occurrence, the overall characteristics of the entire ground against liquefaction were examined in some cases. In other cases, the empiricism was used based on past case studies of whether the soil deposit at a particular condition was subjected to liquefaction during past earthquakes or not. In any case, the conventional assessment of liquefaction occurrence was not based on the dynamic properties of the soil deposit itself concerned.
In the current method of evaluating the dynamic properties of a soil deposit under dynamic cyclic loading, undisturbed sampling from a bore-hole and laboratory triaxial tests on undisturbed samples are required. However, it is extremely difficult to perform soil sampling in an undisturbed manner while preserving soil structures in natural deposits during sampling. It is also noteworthy that soil sampling accompanies the stress relief against the overburden stress, since the soil samples are retrieved from the deep soil deposit to the ground surface. It is therefore difficult to precisely evaluate the in-situ properties of the soil deposit in laboratory tests.
In addition, the undisturbed sampling methods for loose sand deposits, gravel-containing soil deposits, soil deposits involving large-sized grains such as sandy gravels, weathered rocks and soft rocks are not well established, and hence it is not practically possible to perform laboratory tests on such soil deposits.
In the current state of practice, it is therefore found that the range of soils that can be supplied to laboratory tests is extremely limited.